What Is Precast Concrete? Types, Uses and How It's Made

The alternative is concrete cast in situ, poured into formwork where it will stay. Most Irish buildings use some of both, and the question on a real job is which parts go which way. That comparison has its own piece. This one covers what precast is, how it is made, the elements it comes in and when it is the right answer, from an office that designs precast frames for manufacturers and contractors every week.

What is precast concrete made of?

The same ingredients as any structural concrete. What changes is how tightly they are controlled.

• Cement. Portland cement, often blended with ground granulated blast furnace slag (GGBS) or fly ash. Blends lower the carbon in the mix and can improve durability, at the cost of slower early strength, which the factory manages with curing.
• Aggregates. Sand and crushed stone or gravel, graded so the particles pack tightly. In Ireland these are mostly quarried limestone and gravels.
• Water. Kept to a low ratio against the cement, because extra water makes concrete easier to place and weaker once it sets.
• Admixtures. Superplasticisers, which let a stiff, low water mix flow into a mould; accelerators, for early strength; and air entraining agents where the element will see freeze and thaw.
• Reinforcement. Steel bar and mesh, bent and tied into cages, to carry the tension concrete cannot.
• Prestressing strand. High tensile steel wire, stretched before the concrete is poured and released once it has hardened, so the element starts life in compression. Hollowcore floors, and most long beams, are made this way.
• Cast in items. Lifting anchors, connection plates, sockets, ducts for grouted bars, conduits and insulation, all fixed in the mould before the pour so nothing is drilled on site.

How is precast concrete made?

Every element is drawn before it is cast, which is the real difference from in situ work: the engineering, the openings and the connections are settled on paper first. The production itself runs in a fixed order.

1. Detailing. The engineer issues a drawing for each element: dimensions, reinforcement, cast in items, lifting points and the strength it must reach before it can be lifted.
2. The mould. Steel moulds or long casting beds are set up and cleaned, with shutters, openings and recesses fixed to the drawing. A mould used many times is the source of precast's accuracy and of its economy.
3. Reinforcement. Cages are placed with spacers to hold the cover, the depth of concrete over the steel that protects it from corrosion and fire. For prestressed elements, strand is run the length of the bed and tensioned against fixed anchorages.
4. Casting. Concrete from the factory's batching plant is placed and compacted by vibration, or the mix is designed to compact itself.
5. Curing. The element is kept warm and moist so the cement hydrates properly. Factories usually accelerate this with heated beds, steam or covers, which is what makes a daily casting cycle possible.
6. Demoulding and transfer. Once test cubes confirm the element has reached the strength set on the drawing, it is stripped from the mould. On a prestressed bed the strand is cut at this point and the force transfers into the concrete.
7. Storage, finishing and checks. Elements are stored, finished where they are architectural, checked for dimensions and marked so the erector knows where each one goes.
8. Delivery and erection. Elements are loaded so they come off in erection order, craned from the lorry into position, propped where needed and connected.

What are the main types of precast element?

Five families cover most of what leaves an Irish precast yard. Each is an answer to the same question: what is this part of the building being asked to carry.

Precast concrete slabs and floors

Hollowcore slabs are prestressed planks with voids running their length. Standard units are 1200mm wide, typically 150mm to 400mm deep, and span well past ten metres; The Concrete Centre cites a 16 metre span for office loading. The voids take weight out of the floor and can carry services. Hollowcore went into every block at Santry, with a structural screed over it to tie the floor together.

Filigree and wideslab units are thin precast plates with steel lattice girders standing out of the top. They are lifted onto the walls, propped, and a structural topping is cast over them, so the plate becomes the permanent formwork and the bottom of a solid floor. At the Tonlegee Road nursing home the floors are 100mm wideslab units with a 125mm screed, dropped by 75mm at the en suites so the wet rooms finish flush.

Solid slabs are cast to full depth in the factory, used where a floor is short, heavily loaded or needs no topping.

Precast concrete walls and panels

Solid walls are cast to full thickness and carry load, provide stability as shear walls, or both. In a crosswall frame, the walls between rooms are the structure: they carry the floors and hold the building against wind, which is why hotels, student housing and apartments, all built of repeated cells, suit it so well. The Hotel at Goffs is a three storey example, on precast crosswalls with filigree floors.

Twin wall is two thin precast skins held apart by steel lattice girders. The panel is light to lift, and once it is stood and propped the cavity is filled with concrete on site, so the finished wall behaves as one solid reinforced wall. It is the common choice for basements, water retaining structures and walls that need continuity with what is around them.

Insulated sandwich panels consist of an outer skin, a layer of insulation and an inner structural skin, so the façade, the thermal line and the structure arrive as one element. The external walls at 73 North Wall Quay are a close relative: a twin wall with insulation and an in situ concrete core.

Architectural panels are usually cladding: brick faced, polished, acid etched or textured, hung off the frame rather than carrying it.

Precast beams and columns

Columns take load from beams and floors down to the foundations, and can run several storeys in one piece with corbels cast on to carry the beams. Beams come as rectangular, inverted tee and L shaped ledge beams, whose nib carries the floor units. Double tees are the long span option, used mainly in car parks. Sunderland Royal Hospital car park is this kind of frame: multistorey columns, spine and spandrel beams and screeded hollowcore, braced by precast stair cores and solid walls.

Stairs and landings

A stair flight is cast in one piece, treads and risers together, often with its landings. Craned in as the floors go up, it gives the site a finished stair from day one rather than a temporary one.

Foundations, bridges and infrastructure

Precast piles, pad foundations, retaining walls, culverts, tanks and bridge beams follow the same logic outside buildings. At Seatown Lane a precast deck kept the work off the road and out of the channel, and on the Kerry Central Regional Water Supply Scheme we provided the engineering on the precast elements.

How does precast concrete work as a structure?

A pile of well made elements is not yet a building. Precast works when the connections make the pieces act together, and that is where most of the engineering goes.

1. 1Weight runs down the walls to the foundations.
2. 2Wind pushes on the face of the building.
3. 3Each floor, tied together by its screed, carries the wind across as one plate.
4. 4The shear walls and the core take it down to the ground.
5. 5Ties at every joint stop one lost element from bringing down the next.

• Vertical load runs down through walls or columns, each element bearing on the one below through a grouted joint or bearing pad.
• Floors become diaphragms. A structural screed or topping over hollowcore or filigree ties the floor into one plate that collects wind load and delivers it to the walls.
• Shear walls and cores hold the building up sideways. Precast walls, stair cores and lift shafts, tied to the floors, provide the lateral stability.
• Ties give robustness. Steel ties run through the joints, around the perimeter and into the walls, so the loss of one element does not bring down the next. Part A of the Building Regulations requires buildings to resist disproportionate collapse, and ties are the usual way a precast frame does it.
• Temporary stability is designed too. Until the connections are made and the screed has cured, the frame is held by props, which the erection sequence has to account for.

The Holiday Inn at Dublin Airport shows the whole chain: precast shear walls take the wind, propping holds the frame through erection, and once the screeded deck is complete the floor acts as a diaphragm. The joint between the precast and whatever else is on the job matters as much. At Horgan's Quay, two eleven storey blocks with more than 2,000 precast wall and header units, the precast stands against an in situ structure by another design team, and the load transfer and connections were agreed drawing by drawing.

What does designing a precast frame involve?

Most of the design is done before anything is cast, because an element is expensive to change once it has left the mould. On a precast frame the work runs broadly in this order.

1. Layouts, loads and openings. The designer starts from the architectural layouts, the loads and whatever the mechanical and electrical design needs to pass through the frame.
2. The system. Which walls, cores and floors do which job, how the frame stays stable and where in situ concrete stays in the building. Element sizes and joint positions are set against what a lorry can carry and a crane can reach.
3. Analysis and element design. The frame is analysed for vertical load, wind and robustness, to the Eurocodes with their Irish National Annexes. Each element is then designed for the building it ends up in and for demoulding, transport and lifting, when it is supported differently.
4. Connections and ties. Bearings, grouted joints and ties are designed and drawn, with who designs each one agreed in writing before anything is cast.
5. Erection sequence. The order of lifting, the props and the point at which the frame can stand on its own, coordinated with the contractor and the PSDP.
6. Drawings and schedules. A drawing for every element, and the schedules the factory and the site work from. We issue the model to the design team as IFC and the drawings as DWG and PDF, each checked and signed by a chartered engineer.

What are the advantages of precast concrete?

• Speed. Elements are cast while the site is still digging foundations, and a floor is a working platform the day it lands. More on where the time is actually won in what precast buys on a programme.
• Quality and accuracy. Factory moulds, controlled curing and inspection before dispatch give consistent strength, tight tolerances and finished surfaces.
• Less weather risk. Rain and cold stop pours and slow curing; they cost an erection crew far fewer days. High wind does stop the crane, and frost can hold up grouted joints.
• Durability. Dense, well cured concrete with controlled cover resists corrosion and the freeze and thaw of an Irish winter.
• Fire and sound. Concrete is non combustible, and a solid or twin wall separates rooms acoustically without extra layers.
• A smaller, safer site. Fewer trades, less formwork and less waste on site, which matters on a city plot with nowhere to store anything.

What are the disadvantages of precast concrete?

• Decisions are made early. Openings, services and connections are fixed before casting, and a late change costs a remade element.
• Transport and lifting. Element size is limited by what a lorry can carry and a crane can reach, and a site needs access for both.
• Joints need care. Connections, grouting and sealing are where precast buildings go wrong if they are not detailed and supervised.
• Lead in time. Design and casting happen before anything arrives on site, so the programme starts earlier.

The honest summary: precast pays on repetition and programme. When the building is small, nothing about it repeats, or the design is still moving, in situ is usually the better answer, and we will say so.

Is precast concrete sustainable?

It can be, and the case rests on efficiency rather than on the material being green. Cement is the carbon heavy ingredient, and precast lets that be managed tightly: mixes designed to the element, cement replaced with GGBS, steel moulds reused hundreds of times, and offcuts and returned concrete recycled in the yard. Hollowcore floors use less concrete than an equivalent solid slab. Precast also leaves less waste on site, and elements can in principle be taken down and reused. What it does not do is make concrete low carbon by itself: the mix still decides most of the embodied carbon, which is why manufacturers now publish environmental product declarations. The wider picture is in decarbonisation and carbon accounting in Irish construction.

What standards apply to precast concrete in Ireland?

• I.S. EN 13369, the common rules for precast concrete products, which every product standard builds on.
• Product standards for each element family, among them I.S. EN 1168 for hollowcore, I.S. EN 13747 for floor plates, I.S. EN 14992 for wall elements, I.S. EN 13225 for beams and columns and I.S. EN 14843 for stairs.
• CE marking. A precast product covered by a harmonised standard must carry CE marking and a declaration of performance, which is how it meets Part D (materials and workmanship) of the Building Regulations. The new Construction Products Regulation, (EU) 2024/3110, applies from January 2026, with the existing harmonised standards staying valid through a long transition.
• Design to I.S. EN 1992 (Eurocode 2) with the Irish National Annex, including Part 1-2 for fire, with loads and design life from I.S. EN 1990 and I.S. EN 1991.
• Site safety under the Safety, Health and Welfare at Work (Construction) Regulations 2013: lifting plans, temporary stability and a design for erection that the PSDP coordinates.
• Certification. On a job under BCAR, the precast designer's work is carried into the ancillary certificates the assigned certifier relies on.

Precast is the part of our work we are best known for. We design and detail precast frames from the foundations to the roof, modelled in Allplan Precast and analysed in SCIA Engineer, for manufacturers and contractors anywhere in Ireland. If you are weighing it on a project, the precast concrete page sets out how we work.